A non-aqueous electrolyte secondary battery such as a lithium-ion secondary battery has a high energetic density and a characteristic allowing reduction in size and weight of an apparatus. Non-aqueous electrolyte secondary batteries are widely used for main power sources of various electronic apparatuses, power sources for industry or transportation installed in automobiles or airplanes, and main power sources or the like for domestic use.
Normally, in a charge procedure of a non-aqueous electrolyte secondary battery, constant-current charge is performed, and then constant-voltage charge is performed to a full charge state. The charge duration can be reduced by increasing the charge current, but this method increases the charge/discharge cycle degradation of the secondary battery to reduce the service life. While, the charge/discharge cycle degradation can be suppressed by decreasing the charge current, but this method increases the charge duration. In other words, there is a trade-off relationship between the charge duration and charge/discharge cycle degradation with respect to the charge current. For example, when the secondary battery is used as a main driving source in a vehicle or is used as a main power source for domestic use, especially short-term charge is required. However, the short-term charge reduces the service life. When the lithium-ion secondary battery reaches the last stage of the end of life, the available capacity and safety decrease rapidly.
Patent Literature 1 discloses the technology in which the degradation level of a lithium-ion secondary battery is detected, and, when the degradation progresses, full charge is performed at a low set voltage.
Patent Literature 2 discloses the following technology. The actual capacity of a lithium-ion secondary battery is subtracted from the capacity in the brand-new state, the ratio of the subtraction result to the capacity in the brand-new state is calculated as the degradation level. Control is performed so that, as the degradation level increases, at least one of the charge voltage and charge current to be applied to the secondary battery decreases.